When working with transmitters and receivers, designers are forced to deal with the problem of frequency offset between the carrier frequencies in the two parts. In order to allow for inexpensive solutions, one should allow for the use of less accurate frequency references, such as quartz crystals. However, differences between the frequency references in the transmitter and receiver can result in transmission errors. For example, if there is an offset error (dFo) between the transmitter and receiver local oscillators, then the frequencies of received down-converted mark and space signals will be different.